With an appeal to the anxiety about the global environment, the prevention of emission of so-called greenhouse gasses and NOx has been strongly desired. In order to reduce the total emission of these gasses, turning a fuel cell system for automobiles to practical use is considered to be very effective. With the global-level information network becoming very important these days, the fuel cell system is desired also to ensure the energy that is important for the realization of a mobile environment and a ubiquitous society.
A solid polymer fuel cell (PEFC, Polymer Electrolyte Fuel Cell) has an excellent characteristic that only water is generated with a low temperature operation, a high output density, and a power generation reaction. Because a high output can be obtained in a PEFC by pure hydrogen, it is expected to be especially used in automobiles, and hydrogen energy environment maintenance such as a hydrogen station is proceeding. Further, because a PEFC of methanol fuel can be supplied as a liquid fuel similar to gasoline, it is considered to be promising as a power supply source for an electric automobile and a portable apparatus.
Besides the type using pure hydrogen gas, there exist two types of the above-described solid polymer fuel cell of a reformation type in which methanol is transformed into gas having hydrogen as the main component using a reforming device and of a direct type of using methanol directly without using the reforming device (DMFC, Direct Methanol Polymer Fuel Cell). Although the reforming device is necessary in the reformation type, the output is large and the range of application devices is broad. On the other hand, because the reforming device is not necessary in the direct type, it is possible to be made lightweight and there is an advantage such that catalyst poisoning becomes no problem.
A cation exchange membrane is generally used for the electrolyte membrane of the above-described DMFC, and a perfluorocarbon sulfonic acid membrane represented by Nafion (registered trademark) has been mainly used. Although this film is excellent in chemical stability, strength is weak and there is a problem in cell durability because methanol permeates easily and has a high swellability. Further, because it is very expensive, it has been questioned in the side view of practicality.
Because of this, a cation exchange membrane is disclosed that is made by filling the voids of a polyolefin-based porous membrane with a high molecular weight of weight average molecular weight of 500,000 or more with a cation exchange resin (for example, refer to Patent Document 1). Further, in order to reinforce the perfluorocarbon sulfonic acid membrane, an example using a fluorine-based porous membrane as a core material is disclosed (for example, refer to Patent Document 2).
Further, an electrolyte membrane is disclosed in which a mixture of a sulfonic acid group-containing vinyl monomer and a crosslinking agent is impregnated into a crosslinking polyolefin-based porous base and then is polymerized as a membrane in which the ability to suppress methanol permeation of these electrolyte membranes is improved and the output characteristics are consistent (for example, refer to Patent Document 3). In this Document, 2-acrylamide-2-methylpropane sulfonic acid is mainly used as the sulfonic acid group-containing vinyl monomer in the example. However, it was found out that the proton conductivity is insufficient. Further, the case is also disclosed in which a small amount (about 20% by weight) of vinylsulfonic acid is used as a copolymerization monomer in other examples.
However, sodium vinylsulfonate and vinylsulfonic acid that are on the market in general contain impurities such as sodium hydroxyethane sulfonate and hydroxyethane sulfonic acid, and the purity of vinylsulfonic acid is 75% or less. Because of this, even when vinylsulfonic acid is impregnated into a porous base and then the homopolymer or the crosslinking body is polymerized, the polymerization does not proceed sufficiently and a polymer that functions as the proton conductive polymer can not be obtained.
On the other hand, an electrolyte membrane is disclosed in which the permeation of hydrogen gas is suppressed using perfluorocarbon sulfonic acid (for example, refer to Patent Document 4). However, because the hydrogen gas permeability coefficient of this electrolyte membrane is about the same value as that of a perfluorocarbon sulfonic acid membrane represented by Nafion (registered trademark), it cannot be mentioned that the permeation is sufficiently suppressed. Furthermore, the output of fuel cell obtained using the electrolyte membrane is about 600 mW/cm2, and it cannot be mentioned that sufficient cell characteristics are obtained.
Further, an example of the electrolyte membrane is disclosed in which a porous base is filled with an aromatic polymer and then a sulfonation is performed as a membrane in which the ability to suppress hydrogen gas permeation of these electrolyte membranes is improved and the proton conductivity is consistent (for example, refer to Patent Documents 5 and 6). However, the permeability coefficient of hydrogen gas is reduced to about ⅔ to ½ compared with a perfluorocarbon sulfonic acid membrane, and it cannot be not mentioned that the permeation of the hydrogen gas is sufficiently suppressed.
As described above, the electrolyte of PEFC is requested to have 1) impermeability of hydrogen, methanol, etc. (hydrogen and methanol don't permeate the electrolyte), 2) durability and heat resistance, 3) no or less change in the area due to liquid wetting and drying to the membrane at the start and the finish, and 4) proton conductivity, and 5) chemical resistance, etc. However, production of the electrolyte membrane that sufficiently satisfies these requirements has not been developed.
Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No. 01-22932
Patent Document 2: U.S. Pat. No. 5,635,041
Patent Document 3: JP-A No. 2004-146279
Patent Document 4: JP-A No. 06-342665
Patent Document 5: JP-A No. 2001-135328
Patent Document 6: JP-A No. 11-310649